The primary field of application for this invention is guidance of unmanned aerial and underwater vehicles. In such applications it is common practice to compute a guidance command that results in a rate of turn that depends on a measurement of heading error. Both the measurement and the command are with respect to an inertial coordinate frame of reference. The guidance command is received and acted upon by a system (commonly referred to as an autopilot or stability augmentation system), which in turn sends commands to the vehicles servos and/or reaction control devices. This results in a movement of the vehicle's control surfaces, or vectors the thrust produced by the vehicle's propulsion system, or turns on-and-off a reaction control system, or a combination of such means of vehicle control. However vehicles respond to servo command relative to the medium through which they move, and when that medium is also in motion, then the rate of turn that is achieved depends nonlinearly on the speed and direction of the medium through which the vehicle is moving. This dependence can result in degraded or even unstable responses to the guidance system command, particularly for vehicles whose speed relative to the medium is not far greater than the speed of the medium itself (e.g. guided parafoils and underwater vehicles).
One approach to this problem is to sense the vector motion of the vehicle relative to the medium through which it is moving as well as its inertial velocity vector. By differencing these two quantities, one can compute the inertial velocity of the medium and provide a correction to the guidance command for that motion. However this requires incorporating devices that sense the vehicle's vector relative motion, which are not normally a part of the existing guidance and control system design. Such sensors in an aircraft are often referred to as an air data system.